6N6 Line Stage
For Xmas 2017 I got Sennheiser HD650 headphones and needed an amplifier to drive them.
The project was documented here
After a bit more experimentation, I arrived at the circuit below.
Initialy the design was almost a copy of Lynn Olson’s Raven. That design as Lynn says, is very similar to a Western Electric line driver but with modern ideas and parts.
I found it fairly immune to part types and although the 100u capactitors are Black Gate Wkz, I couldn’t tell reliably between the BG Wkz and a BHC electrolytic. During resistor testing, in other stages of the phono and GM70 amps, differences in these types of stages in the power supply were hard to detect. I think that proves differential circuits are good at power supply rejection leaving the sonic signature largely down to valve selection.
I started with a Sowter 3575 at the input but preferred the LL1544a. The output transformer I prefer configured for 4.5:1. If for some reason in the future I need to drive lower impedance headphones, there is 9:1 and 18:1 available.
The stage runs flat out with the output attenuated by the TVC. I’m using it as an AVC ignoring the primary. I think in this situation, it works better and certainly prefered to it being at the input.
I crammed it into the passive preamp case I had which used to only house the selector switch and TVC. It’s theusual breadboard in a box. Plan is to rebuild with a slide in/out arrangement like the phono. Amazingly it is super quiet.
Inside Line stage
This has now morphed into the 12B4 Line stage